1. Field of the Invention
The present invention relates to a method of measuring a phase difference and an apparatus for carrying out such a method. For instance, in a photolithography used in a manufacture of semiconductor devices, photomasks and reticles are used as originals for pattern projection. These photomasks and reticles are checked by such phase difference measuring method and apparatus. Particularly, a phase difference introduced by a photomask having a phase shift region within a pattern is measured,
2. Related Art Statements
A degree of integration of semiconductor devices has been increased and a semiconductor device manufacturing technique has required a higher definition. After the reduction exposure method has been developed, the precise working has been advanced. Recently there have been proposed various methods of increasing a resolution of a projected pattern image, such as a modified illumination method and phase shift method.
In the phase shift method, a phase shift film is provided on a photomask in order to introduce a phase difference. There have been proposed a Levenson type photomask in which a phase shift film is applied on a pattern and a half-tone photomask in which a pattern has a phase difference as well as a half-tone, both of said photomasks performing similar functions. In either case, upon checking the photomask, it is required to measure a phase difference introduced by the phase shift film. However, heretofore there has not been established any effective method for measuring the phase difference.
A phase difference may be measured by various means such as a Mach-Zehnder interferometer, a Michelson interferometer, a film thickness measuring method by spectroscopy, a heterodyne method and a Nomarski method.
In the Mach-Zehnder interferometer and Michelson interferometer, a light flux is divided by a beam splitter into a reference light flux and a measuring light flux, a photomask without a phase shift film and a photomask with a phase shift film are inserted in the reference light flux and measuring light flux, respectively, and an interference image produced by the phase difference is analyzed.
In the film thickness measuring method, a thickness of a phase shift film is measured with the aid of a spectrometer and ellipsometer, and an introduced phase difference is estimated from the measured thickness. In the heterodyne method, an interference image due to the phase difference is analyzed by a method similar to the interference method.
The heterodyne method has been described in "Transmission and Reflection Phase Measurement with Differential Optical Heterodyne Method Takeno Ode et al, " Japan Society of Laser Microscope, 11th Meeting, Themes 1993, An example of the heterodyne method has been disclosed in "Direct Phase Measurement in Phase Shift Masks with Differential Heterodyne Interferometer", Hiroshi Fujita et al, 54th Applied Physics Meeting Preliminary Document No. 2, 28a-SHF-22. In this method, two light fluxes having a slightly different frequencies are laterally shifted on an optical axis with the aid of an acoustic-optical (AO) element, and light fluxes transmitted through mask portions with and without a phase shifter are detected as a heterodyne beat signal on a photodiode. The above lateral shift may be adjusted by a frequency modulation of the AO element.
The above mentioned Nomarski method has been described in, for instance Japanese Patent Application Laid-open Publication Kokal Hei 4-151662; "Phase Shift and Image Quality Measurement with UV Light in Photomask Evaluation", Tsuyoshi Fujiwara et al, 41st Applied Physics Joint Meeting Preliminary Document No. 2 28p-MV-14; and "Interferometer for phase measurements in phase shift masks", Derek B. Dove et al (SPIE) Vol. 1809, p. 128-136, 1992.
In the above mentioned Kokai Hei 4-151662, there is explained a Normaski observing method used in a microscope, in which a linearly polarized beam having a wavelength which is identical with that of an exposing light flux is divided by a birefringent prism and condenser lens into two light fluxes which have different polarized components and are shifted laterally from each other. These two light fluxes are transmitted through a photomask and an objective lens system and are combined again by means of a birefringent prism. A phase difference adjusting means is arranged in this optical path, and an intensity of interference of portions corresponding to the phase shift film of the pattern image is measured, In this case, a lateral shift separated on the photomask corresponds to a phase difference between a transparent portion and a phase shift portion of the photomask, so that the phase difference can be measured by detecting an intensity of the interference.
Fujiwara et al utilizes the same optical system as that disclosed in the above Kokai Hei 4-151662 and a phase shift is measured by using a combination of a phase shift interference method utilizing the phase difference adjusting means and a phase modulation method due to a movement of a Nomarski prism.
Derek D. Dove et al have proposed the phase shift interference method by a voltage modulation using an eletro-optical crystal as the phase modulating means, while a laser light source is used in a basic construction which is similar to that of the above mentioned Kokai Hei 4-151662.
When a half-tone mask is checked by using the above mentioned known phase difference measuring methods and apparatuses, a density of a relevant region is changed in accordance with a content of a pattern, so that there might be generated a remarkable difference between the reference light flux and the measuring light flux. This results in an large decrease in a contrast of interference fringes, and in a worse case, the interference fringes might be buried under one of the light fluxes and could not be detected at all.
In the Mach-Zehnder method and Michelson method, the interferometers are liable to be influenced by mechanical vibration and a optical path difference of the two light fluxes has to be controlled or adjusted on a wavelength level. Further, in order to perform a correct measurement, the photomask and objective lens system should have the same transmission wave front on the reference light flux path and measuring light flux path except for the phase shift film. The heterodyne method has similar problems, and further has a problem that adjustment of optical elements is difficult, so that the measurement could not be performed easily. In the thickness measurement method, the phase difference has to be estimated from the measured thickness of the phase shift film, so that there might be introduced a relatively large error.
In the above mentioned heterodyne method proposed by Ode, there are problems in stability of laser mode, isolation of the AO element and phase characteristic of a signal receiving amplifier. Therefore, adjustment of an actual apparatus is very critical and the measurement could not be carried out simply. Moreover, it is rather difficult to obtain a laser light source emitting stably a laser light beam having the same wavelength as that of the pattern exposing light such as i-ray. Therefore, a phase difference at i-ray has to be presumed from a measured phase difference using other ray.
In the known Nomarski method disclosed in the above mentioned Kokai Hei 4-151662, a phase difference is derived from an amount of a phase difference adjustment between the two light fluxes at which an intensity of the interference image due to the phase shift region in the pattern becomes maximum and minimum. However, a difference between the maximum intensity and the minimum intensity is very small, so that a phase difference adjustment amount could not be determined precisely. Therefore, it is very difficult to set a threshold value for the phase difference adjustment amount, and a precise measurement could not be performed and human error is liable to be introduced.
In the phase shift interference method using the Nomarski prism disclosed in Fujiwara et al and D. B. Dove et al, an amount of separation of the two light fluxes due to the Nomarski prism corresponds to a pitch of the transparent portions and phase shift portions on the reticle. When a pattern includes a relatively large pitch, an amount of separation should be made large accordingly, so that a volume of the Nomarski prism becomes inherently large and the prism could not be easily installed in known microscopes. When an amount of separation is large, there might be generated adjacent double images, which affects the measurement.
Fujiwara et al have tried to perform the separation and the phase difference adjustment simultaneously by moving the Nomarski prism, however the prism is larger than a prism usually provided in a differential interferometer, and thus the movement of the prism within a small range could not be carried out precisely.
D. B. Dove et al have proposed to use the phase modulating method using the voltage modulation for the electro-optic crystal, so that a quick response can be attained and a static modulation can be performed. However, in the electro-optical crystal, a range over which the phase difference between the two light fluxes can be introduced could not be made wide, so that the application to the reticle is limited by a maximum allowable amount of phase shift as well as by the electro-optic crystal.
The Nomarski method can be applied to the Levenson type photomask, but could not be applied to the half-tone type photomask, because in the half-tone photomask, there is produced a difference in intensity between the reference light flux and the measurement light flux.